Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Identifiable assets	$ 14,105.3	$ 13,443.7
Identifiable assets	107.4	110.2
Property and Casualty Insurance Companies [Member]
Segment Reporting Information [Line Items]
Identifiable assets	13,997.9	13,333.5
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 107.4	$ 110.2